Optical polarizing films with designed color shifts

ABSTRACT

Multilayer films are provided that exhibit a colored appearance when viewed at an oblique angle as a result of one or more reflection bands in the visible region of the spectrum. The films however provide no substantial reflection bands in either the visible or near infrared regions for light normally incident on the film. The films can be made to shift from clear at normal incidence to an arbitrary designed color at an oblique angle without necessarily becoming cyan.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 11/424,711, filed Jun.16, 2006, and now issued as U.S. Pat. No. 7,256,936 (Hebrink et al.),which is a continuation of U.S. application Ser. No. 10/335,460, filedDec. 31, 2002, and now issued as U.S. Pat. No. 7,064,897 (Hebrink etal.).

FIELD OF THE INVENTION

The present invention relates to optical films. More particularly, thepresent invention relates to optical films whose apparent color changesas a function of viewing geometry.

BACKGROUND

Optical films that exhibit a visible color shift as a function ofviewing geometry are known. See, e.g., PCT Publication WO 99/36258(Weber et al.) entitled “Color Shifting Film”. See also U.S. Pat. No.6,045,894 (Jonza et al.) entitled “Clear to Colored Security Film”.These references disclose many different films, each of which exhibits ashift in apparent color as the observation or incidence angle θ(measured from the surface normal) changes. Filters that comprise aglass or other rigid substrate having a stack of inorganic isotropicmaterials deposited thereon can also exhibit color shifts.

A common feature of these films is the presence of one or morereflection bands for normally incident light (θ=0), which band(s) thenshift to shorter wavelengths as θ increases. The physics of thisso-called “blue shift” of the reflection band can be explained inconnection with FIG. 1, where a portion of a multilayer film 10 is showngreatly enlarged. A light ray 12 is incident from medium 1 (withisotropic refractive index n₁, for simplicity) at an angle θ₁. Part ofthe light ray reflects at an upper interface 14 between medium 1 andmedium 2, and another part reflects at a lower interface 16 aftertraversing the layer of medium 2, whose physical thickness is d. Medium2 is also assumed to have an isotropic refractive index, n₂, forsimplicity. The two reflected rays 18, 20 eventually constructively ordestructively interfere depending on the relative phases of the rays.The relative phase in turn is a function of the optical path difference(OPD) between the rays, given by:OPD=2·n ₂ ·d·cos(θ ₂)  (EQ. 1)This quantity decreases with increasing incidence angle, correspondingto a shift to shorter wavelengths. Although the analysis is morecomplicated, multilayer optical films that have at least some opticallayers that are birefringent rather than isotropic also experience ablue shift with increasing angle.

As the band(s) shift to shorter wavelengths, they also each split intotwo distinct bands: one for s-polarized light, the other for p-polarizedlight, where s-polarized light refers to linearly polarized light whoseelectric field vector oscillates perpendicular to the plane ofincidence, and p-polarized light refers to linearly polarized lightwhose electric field vector oscillates parallel to the plane ofincidence. The shift to shorter wavelengths can also be accompanied by ashift in the spectral width and shape of the reflection band, andchanges in the out-of-band and in-band reflectivity. The amount ofblue-shift one can attain is limited, and is a function of the medium inwhich the film is immersed, and the details of the film construction.

The observed color change of these known films is a manifestation of theshift of the reflection band(s) to shorter wavelengths. Since thehuman-visible region corresponds to a segment of the electromagneticspectrum extending from about 400 to 700 nm, a film that is clear (i.e.,substantially colorless) at normal incidence can become colored atoblique angles only by the shifting of a reflection band whose positionat normal incidence is somewhere in the near infrared region, i.e., ator above about 700 nm. As this band begins to move into the visibleregion with increasing observation angle, it begins to block longvisible wavelengths in the red, thus giving rise to a cyan appearance intransmission. This is shown schematically in FIG. 2, where a reflectionband 30 a for normally incident light is located initially in the nearinfrared region of the spectrum, and then as the angle of observationincreases it transforms into band 30 b at shorter wavelengths, and withincreasing observation angle transforms into band 30 c at still shorterwavelengths. (In FIG. 2, spectral ringing and separation into distincts- and p-polarization reflection bands are ignored for ease ofexplanation.)

The spectral position of the reflection band at normal incidence is setby the optical thickness of the optical repeat units in the film. Theoptical thickness of a layer refers to its physical thickness multipliedby the relevant refractive index of light. Optical repeat unit refers toa stack of at least two individual layers that repeats across thethickness of a multilayer optical film, though all repeating layers neednot have the same thickness. As an example, known clear-to-colored filmsreflect normally incident light from approximately 720 to 900 nanometersby utilizing optical repeat units whose optical thicknesses range from360 to 450 nanometers (half the wavelength of the light desired to bereflected).

It would be advantageous to have at the optical designer's disposalfilms that could exhibit human visible color shifts other than thosecaused by a simple blue shift of existing reflection bands. Further, itwould be advantageous to have available a film that could transitionfrom clear at normal viewing to any desired color at an oblique angle.

BRIEF SUMMARY

The present application discloses films whose apparent color change withangle does not require the presence of a reflection band for normallyincident light that then simply shifts to shorter wavelengths withincreasing incidence angle.

In one aspect, the present specification discloses an optical filmhaving a plurality of layers effective to provide a reflection bandcovering a portion of the visible spectrum at an oblique angle such thatthe optical film appears colored at such oblique angle. However, theplurality of layers provide substantially no reflection bands fornormally incident light.

In another aspect, the specification discloses an optical film havinglayers that form a plurality of optical repeat units. At least some ofthe optical repeat units have optical thicknesses equal to half of awavelength of visible light, yet the optical film has a clear appearanceat normal incidence.

In still another aspect, the specification discloses optical films whosetransmitted appearance changes from substantially clear to a first colorover an angular range of observation angles. At least some of theseoptical films, however, do not appear cyan over such angular range.

These and other aspects of disclosed embodiments will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on claimed subject matter, whichsubject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification, reference is made to the appendeddrawings, where like reference numerals designate like elements, andwherein:

FIG. 1 is a cross-sectional view of a portion of a multilayer opticalfilm;

FIG. 2 is a schematic representation depicting the blue shift of areflection band of a PRIOR ART optical film;

FIG. 3 is a schematic representation depicting the emergence of areflection band in a portion of the visible region according to thepresent description;

FIG. 4 is a cross-sectional view of a portion of a multilayer film asdescribed herein;

FIG. 5 is a perspective view of a film as described herein that has beenembossed or otherwise thinned in a portion thereof, FIG. 6 is a plot ofphysical thickness of all 275 optical layers in an example filmdescribed below;

FIGS. 7 a and 7 b show the measured transmission of an example film inair at normal incidence and 60°, respectively;

FIG. 8 shows a calculated transmission spectrum for p-polarized light atabout 60° incidence of two optical layer packets that can be used in asingle optical film to yield a green transmitted color at such angle;and

FIG. 9 shows a calculated transmission spectrum for p-polarized light atabout 60° incidence of a dyed optical film.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

To highlight a difference between previously known color shifting filmsand color shifting films as described herein, FIG. 3 provides aschematic representation of a reflection band for a film such asdescribed herein. For normally and near-normally incident light, a curve32 a is provided merely to show that there is essentially no reflectionband at all. As explained below, a slight in-plane mismatch ofrefractive indices between individual layers may give rise to a barelyperceptible reflection band, but the reflectivity of such band isgenerally less than 20%, more typically less than 10% or 5%.(Reflectivity values given herein assume illumination with unpolarizedlight unless otherwise noted. The reflectivity of a reflection band isunderstood herein to be the maximum reflectivity of such band exclusiveof any outer surface reflections of the film.) As the observation angleincreases, the reflection band appears in the visible region andincreases in strength as shown by curve 32 b. The reflection bandincreases still further with increasing observation angle as shown bycurve 32 c. The peak reflectivity of the reflection band thus increasessubstantially monotonically with increasing θ, even though slightdecreases in peak reflectivity of a few percent may in somecircumstances occur with increasing θ at low reflectivity values if thein-plane refractive index mismatch is significant. Although a blue shiftcan be seen in the sequence of FIG. 3 (as was the case in the sequenceof FIG. 2), FIG. 3 is most clearly distinguished by the fact that thereflection band essentially emerges from nowhere, rather than simplyshifting over to the left. By judicious selection of the layerthicknesses in the optical film, one or more reflection bands can bemade to appear anywhere in the visible spectrum at a selected obliqueangle in a manner like that shown in FIG. 3, thus allowing the film toshift to any desired transmitted color at a selected oblique angle.Clear-to-green, clear-to-yellow, clear-to-magenta, clear-to-red, andclear-to-blue are examples of color shifts that are possible. A film canbe considered clear if, having CIE color coordinates a* and b*, each areno greater than 5, or, more stringently, if the square root of a*²+b*²is no greater than 5. Note that although a great many embodiments existwhere the film does not appear cyan over the useable range of entranceangles, in some embodiments the film may well appear cyan at someentrance angles, if it is so designed. If absorbing agents are added tochange the baseline on-axis appearance from clear to a particular color,still further transitions are possible such as yellow-to-red orblue-to-green.

The reflection band of FIG. 3 is associated with p-polarized light, nots-polarized light. The film is therefore a polarizing film at obliqueangles. S-polarized light passes through the film without substantialreflection (except for possible outer surface reflections, which are notconsidered since they are substantially wavelength insensitive). Becauseof this, the reflection band of FIG. 3 can achieve a maximumreflectivity of 50% for unpolarized light, and the color saturation ofthe film (when illuminated with ordinary unpolarized light and viewed intransmission) will not be as great compared to a film that can filterboth polarizations. Stated differently, the apparent color saturationcan be greatly enhanced if the film is illuminated with only p-polarizedlight, or if it is observed through an analyzer that transmits onlyp-polarized light. Conversely, the colored appearance of the film can beessentially eliminated even at highly oblique angles if the film isilluminated with only s-polarized light, or if it is observed through ananalyzer that transmits only s-polarized light. Significantly, theappearance of the film (whether colored or not, depending on theillumination and viewing conditions) is insensitive to rotations of thefilm about an axis perpendicular to the plane of the film, and torotations of the observer about the film in such a way that theobservation angle θ is maintained constant.

The optical properties just described can be achieved with a multilayerinterference film construction with appropriate selection of layerthicknesses and refractive indices. In a simple quarter-wave stackconstruction, the film comprises alternating layers of a first andsecond light transmissible material A, B, which layers have opticalthicknesses equal to one-fourth of the visible wavelength to bereflected. A pair of adjacent A,B layers then form an optical repeatunit whose optical thickness is ½λ. A variation of this is where theoptical thicknesses of the layers are not equal, or in other words thef-ratio is different than 0.50. This more general situation is shown inFIG. 4. There, an optical film 40 is provided with alternating layers Aand B that form six optical repeat units ORU1, ORU2, . . . ORU6. Thesehave corresponding optical thicknesses OT1, OT2, . . . OT6 whichindividually are the sum of the optical thicknesses of the applicable Alayer and the adjacent B layer. Although only six optical repeat unitsare shown, typical films can include tens, hundreds, or thousands ofindividual layers. The optical repeat unit thicknesses can all be equal,in which case a relatively narrow reflection band is produced, or theycan be different such as is the case with a linear gradient in layerthickness along the thickness axis of the film, producing a widerreflection band. Other layer thickness gradients can also beincorporated, such as described in U.S. Pat. No. 6,157,490 (Wheatley etal.), “Optical Film With Sharpened Bandedge”.

Film 40 is shown in the context of a local x-y-z right-handed Cartesiancoordinate system, where the film extends parallel to the x-y plane, andthe z-axis is perpendicular to the film, corresponding to a thicknessaxis. The refractive indices of the individual A layers are denoted:

-   -   n₁ x, n₁ y, n₁ z        for polarized light whose electric field vector oscillates along        the x-, y-, and z-axes respectively. In like fashion, the        refractive indices of the individual B layers are denoted:    -   n₂ x, n₂ y, n₂ z.        Although strictly speaking the refractive index of most light        transmissive materials is also wavelength dependent, such        dependence is typically very small, particularly within the        visible region, and will be ignored here. In order to achieve        the optical properties discussed above, at least one of the A        and B layers within each optical repeat unit is birefringent,        such that there is a substantial match of refractive indices of        adjacent layers along the in-plane axes, and a substantial        mismatch of refractive indices along the thickness axis. If we        denote the magnitude of n₂−n₁ along a particular axis as Δn,        this set of conditions can be expressed as:    -   Δn_(x)≈0    -   Δn_(y)≈0    -   Δn_(z)≈large        The resulting film is referred to as an “off-axis polarizer” or        a “p-polarizer”. See generally U.S. Pat. No. 5,882,774 (Jonza et        al.), “Optical Film”. In the relationships shown above, zero for        Δn_(x and for Δn) _(y) means the difference is sufficiently        small to produce a negligible amount of on-axis (θ=0)        reflectivity for either polarization, e.g. less than about 20%,        or 10%, or even 5%. This will depend on the total number of        optical repeat units employed in the film, with a larger number        of optical layers or optical repeat units generally requiring a        smaller absolute value of the in-plane index difference to        maintain a low reflectivity, and also on the thickness        distribution (or “layer density”—the number of layers per range        of optical thickness) of the optical repeat units. For a film        having a total number of optical layers of a few hundred but        less than one thousand, a refractive index difference of up to        about 0.02 is typically acceptable, but a difference of 0.01 or        less is preferred. “Large” for Δn_(z) means large enough to        produce a desired substantial amount of off-axis reflectivity,        preferably at least 50% and more desirably at least 80%        reflectivity for p-polarized light. These levels are achieved at        oblique angles θ (measured in an air medium) of typically 50 to        80 degrees, preferably about 60 degrees. A preferred value for        Δn_(z) is about 0.1 or greater. The greater the value of Δn_(z),        the greater the reflectivity each optical repeat unit in the        optical film for p-polarized light at a given oblique angle, and        the greater the reflectivity of the film for a fixed number of        optical repeat units, or the fewer optical repeat units required        in the film for a desired reflectivity level. Reference is made        to commonly assigned U.S. Application Publication 2004/0126549        (Ruff et al.), filed Dec. 31, 2002.

A multilayer film with these refractive index relationships exhibitsessentially no reflection bands for normally incident light. This isbecause the electric field vector of such light oscillates only alongthe in-plane axes, thereby sampling only the in-plane refractiveindices. Since those indices are substantially matched fromlayer-to-layer, the light beam behaves as though traveling through amonolithic material with no internal interfaces. It is only when thelight propagates at a substantial angle to the z-axis, and then onlywhere the electric field vector has a component along the z-axis(p-polarized light), that a substantial refractive index difference isexperienced by the light at the individual layer interfaces, thus givingrise to reflection by constructive interference.

In addition to the refractive index relationships discussed above,however, the optical repeat units should have optical thicknesses thatproduce at an oblique angle a reflectivity over the human visiblespectrum that is non-uniform, so that the film exhibits a coloredappearance in transmitted light at such angle. The optical thicknessesof the optical repeat units can be chosen to all be equal such that asingle, relatively narrow reflection band emerges in a desired portionof the visible spectrum with increasing incidence angle. Alternatively,multiple packets of optical repeat units can be used, where each packethas optical repeat units of a uniform optical thickness, but suchoptical thickness being different for the different packets so thatdistinct narrow reflection bands emerge in a desired part of the visiblespectrum. Alternatively or additionally, thickness gradients can beemployed to produce broadened reflection bands over portions of thevisible spectrum. Multiple reflection bands can be separated by asufficient degree to define a spectral region of high transmission (atransmission band) therebetween over a desired wavelength band such asblue, green, or red. Appropriate selection of the thicknesses of theoptical repeat units therefore give the designer wide latitude toachieve nearly any desired color appearance at the oblique observationangle, not only cyan, even for a film that is substantially clear atnormal incidence.

The reflectivity of a given optical repeat unit exhibits a maximum at awavelength X equal to two times the optical thickness of the opticalrepeat unit, at normal incidence. For purposes of the presentapplication, the optical thickness of an optical repeat unit isconsidered to be a constant, and equal to the sum of the opticalthicknesses of the optical repeat unit's constituent optical layers fornormally incident light. At least some (and preferably substantiallyall) of the optical repeat units in the subject films reflect visiblelight over a range of nonzero angles of incidence, i.e., over a range ofoblique angles of incidence. Therefore, although the reflection band atlarge incidence angles is blue-shifted to some extent relative to thereflection band at small incidence angles, most optical films describedherein will nevertheless have at least some optical repeat units whose(normal incidence) optical thickness is equal to half of a wavelength ofvisible light, or half of a wavelength between about 400 and 700 nm, orfrom about 200 to 350 nm, while also having a normal angle transmittedappearance that is substantially clear and/or having substantially noreflection bands at normal incidence, whether in the visible or nearinfrared regions.

As discussed above, each optical repeat unit can consist essentially ofjust two light transmissible optical layers. The reader will appreciatehowever that other known optical repeat unit designs can also be used inaccordance with the above teachings. For example, four layer designsusing three different materials as described in U.S. Pat. No. 5,103,337(Schrenk et al.), “Infrared Reflective Optical Interference Film”, andsix layer designs using two materials as described in U.S. Pat. No.5,360,659 (Arends et al.), “Two Component Infrared Reflecting Film”, canalso be used. In most instances, however, a simple two-componentquarter-wave (0.50 f-ratio) design is preferred since it provides highreflectivity for the lowest order reflection and since higher orderreflections are generally of no concern.

A variety of light transmissible materials can be used for the opticallayers making up the optical repeat units of the subject films.Preferably, however, the materials are thermoplastic polymers that canbe co-extruded from a multilayer die and subsequently cast and orientedin sequential or simultaneous stretching operations. Optically thickskin layers can be added for protection and ease of handling, whichlayers can become protective boundary layers between packets of opticallayers within the finished film if one or more layer multipliers is usedbetween the feedblock and the die.

In one approach that has been found advantageous, one lighttransmissible polymeric material (arbitrarily designated A) remainsisotropic throughout the manufacturing process, and another (arbitrarilydesignated B) becomes birefringent during a stretching procedure in themanufacturing process. The stretching is carried out along both x- andy-axes so that the in-plane refractive indices of the birefringentmaterial end up being about equal to each other, and equal to therefractive index of the isotropic material. The out-of-plane refractiveindex of the birefringent material then differs substantially from therefractive index of the isotropic material. In a particularly preferredversion of this approach, material A has a relatively high (isotropic)refractive index and material B has a somewhat lower isotropicrefractive index in the cast film before orientation. During orientationthe refractive indices of the B material increase along the twoorthogonal stretch directions to match the index of the A material, andthe z-axis refractive index of the B material diminishes to widen thegap between it and the index of the A material. Meanwhile, withappropriate materials selection and careful control of the stretchconditions such as film temperature, stretch rate, and stretch ratio,the refractive index of the A material remains constant and isotropicduring orientation. Material A has a high refractive index to match thein-plane refractive indices of the oriented material B, and a low enoughglass transition temperature T_(g) to remain isotropic when oriented atconditions necessary to cause birefringence in material B. Preferably,the film is maintained at a temperature of at least about 20° C. abovethe glass transition temperature of the isotropic material duringstretching.

For design flexibility, conventional absorbing agents such as dyes andpigments can be added to one or more layers of the film, or can beapplied in one or more coatings such as an adhesive, ink, or hard coat,or incorporated in a separate film or substrate that is subsequentlylaminated to the subject multilayer optical films, to add a baselinecolor or tint to the film or article for visual effect. This baselinecolor would of course be effective at essentially all viewing angles.Additional layers and coatings can also be added to modify optical,mechanical, or chemical properties of the film. See U.S. Pat. No.6,368,699 (Gilbert et al.), “Multilayer Polymer Film With AdditionalCoatings or Layers”. Conventional multilayer films and polarizers otherthan p-polarizing films can also be laminated to or otherwise used withthe films described herein. Such conventional films may have reflectionbands in the visible and/or near infrared regions of the spectrum foraesthetic and/or utilitarian purposes.

The unique appearance characteristics of the subject films can befurther modified by selectively thinning portions of the film to definea feature, pattern, or indicia. Such selective thinning preferablyinvolves more than simply thinning a skin layer or coating, but ratherthinning all the optical layers through the thickness of the film at thelocalized positions so that the perceived color at oblique angles ischanged at those positions. This can be done by localized heating,embossing, or exposure to suitable laser radiation. Preferably thethinning is done after the desired refractive index relationships areestablished through the orientation process. In that way both thethinned portions and the remaining portions exhibit the desirablerefractive index and wavelength properties described above. An exampleis shown in FIG. 5. There, portion 52 has been thinned in the form of acorporate logo on an optical film 50 that also includes unthinnedportion or background 54. In both portions, the in-plane refractiveindices of adjacent optical layers are substantially matched, and theout-of-plane refractive indices of such layers are substantiallymismatched. When viewed normally along the z-axis, no pattern isdiscernable since both portions substantially transmit normally incidentlight. The film 50 can be completely clear, or of a uniform color ifabsorbing agents are present. At an oblique angle, however, theunthinned portion 54 changes to a first transmitted color and thinnedportion 52 changes to a second transmitted color that is blue-shiftedrelative to the color of portion 54, the amount of blue-shift beingproportional to the degree of thinning of the optical layers. Hence, thepattern is difficult to detect at normal viewing but becomes clearlyvisible at oblique angles. The pattern can incorporate more than twoportions, each having a distinct thickness and hence a distinct color atoblique angles, and can also incorporate a gradual thickness change fromone portion of the film to the other rather than step changes.

Alternatively, indicia can be added to the films by localized surfaceroughening or texturing. Such roughening scatters both s- andp-polarized light, and roughened regions stand out from the surroundingoptical film. Localized surface texturing can be achieved by a varietyof known techniques, such as laser marking, sandblasting, embossing witha matte finish roll, rubbing, and impinging jets.

Films as described herein and articles incorporating such films can beused in a variety of end-use applications. For example, specializedoptical systems can benefit from the unique properties of a p-polarizer.See, for example, commonly assigned U.S. Pat. No. 6,952,312 (Weber etal.). Another end-use application is the area of authentication systems.The subject films can be permanently affixed to a document, such as apassport, so that an observer can read the document through the film,but can also tell whether the document is authentic by observing theunique color shift at oblique angles, optionally with an analyzingpolarizer or with polarized light. The document or other substrate overwhich the film is applied can include indicia that are colored in such away that the transmitted color of the film at an oblique angle matchesthe color of the indicia making them difficult to read, while they areeasily read at normal incidence. The films can be sold in the form of atape or label, which can be adhesively secured to a document or to apackage for consumer goods, again for purposes of authentication. Anadhesive—preferably a pressure sensitive adhesive but alternatively ahot-melt or curable adhesive—can be applied to one major surface of thefilm so that it can be applied to an object. The films can also be soldin the form of a security thread to be incorporated into a securitydocument.

Conventional printed images and/or holographic images can be provided oneither major surface of the films, by any suitable technique. Otherconventional security features that can be incorporated into the subjectfilms, or any suitable construction of which the film is a part, includemicroperforations that effectively prevent tampering, heat shrinkcharacteristics that prevent tempering by the application of heat,patterned differential adhesion layers that effectively preventtempering by delamination, and internal delamination characteristicsthat provide an indication of tampering. The subject films can also beincorporated into any suitable label, laminate, or card (such as anidentification card or transparent or translucent financial transactioncard), whether on the surface or in an interior layer of such item.

EXAMPLE

An example film will now be described. The polymer used in the isotropiclayers of the film construction was specially formulated to achieve thenecessary rheological, chemical, thermal, and optical properties. Thepolymers used in the film were chosen and/or developed according to thefollowing conditions: they should be coextrudable; they should haveadequate interlayer adhesion; and the isotropic polymer should have anunusually high refractive index in order to match the in-planerefractive indices of the birefringent polymer after stretching, and alow enough glass transition temperature so that it remains isotropicwhen oriented under conditions necessary to cause birefringence in theother polymer material. Preferably, the film is maintained at atemperature of at least about 20° C. above the glass transitiontemperature of the isotropic material during stretching.

Polymer 1—Co-PEN-HNLT

A copolyester was synthesized in a batch reactor with the following rawmaterial charge: 127.3 kg dimethyl naphthalene dicarboxylate, 4.2 kgdimethyl isophthalate, 38.4 kg hexane diol, 50.5 kg ethylene glycol, 8.6kg 1,3 butyl ethyl propanediol, 1.3 kg trimethylol propane, 34 g zincacetate, 25 g cobalt acetate, and 75 g antimony triacetate. Underpressure of 0.20 MPa, this mixture was heated to 254° C. while removingmethanol. After 34.5 kg of methanol was removed, 56 g of triethylphosphonoacetate was charged to the reactor and then the pressure wasgradually reduced to 133 Pa while heating to 285° C.

The condensation reaction by-product, ethylene glycol, was continuouslyremoved until a polymer with an intrinsic viscosity of 0.84 dL/g, asmeasured in 60/40 wt. % phenol/o-dichlorobenzene at 86° C., wasproduced. This material, a thermoplastic polymer, had a glass transitiontemperature T_(g) of 76° C. as measured by DSC using ASTM D3418 with ascan rate of 20° C./min, and at a relative humidity of about 50%. Thethermal history of the polymer was removed as a factor by performing twoDSC heat scans on the sample and recording the T_(g) of the second heatscan.

Polymer 2—PET

The polyethylene terephthalate used in the example is synthesized in abatch reactor with the following raw material charge: 5,000 kg dimethylterephthalate, 3,502 kg ethylene glycol, 1.2 kg manganese acetate, and1.6 kg antimony triacetate. Under pressure of 1520 torr, this mixture isheated to 254° C. while removing the transesterification reactionby-product methanol. After 1,649 kg of methanol is removed, 2.45 kg oftriethyl phosphonoacetate is charged to the reactor and then thepressure is gradually reduced to 1 torr while heating to 280° C.

The condensation reaction by-product, ethylene glycol, is continuouslyremoved until a polymer with an intrinsic viscosity of 0.60 dL/g, asmeasured in 60/40 wt. % phenol/o-dichlorobenzene at 86° C., is produced.This material, a thermoplastic polymer, has a glass transitiontemperature T_(g) of 79° C. and a melting temperature T_(m) of 255° C.as measured by DSC using ASTM D3418 with a scan rate of 20° C./min, andat a relative humidity of about 50%. The thermal history of the polymeris removed as a factor by performing two DSC heat scans on the sampleand recording the T_(g) of the second heat scan.

Polymer 3—PETG

This copolyester was obtained commercially from Eastman ChemicalCompany, Kingsport, Tenn., under product code Eastar brand PETG 6763. Itexhibits a glass transition temperature T_(g) of 83° C.

Polymer 4—70/30 Polyester Blend

This is a blend of 70 wt % PET and 30 wt % PETG. It exhibits a glasstransition temperature T_(g) of about 81° C.

Film Example

A multilayer optical polarizing film was made using Polymer 1 as one ofthe light transmissible materials, and Polymer 4 (the blend of 70 wt %PET and 30 wt % PETG) for the other material. These materials werecoextruded through a multi-layer melt manifold to create a stack of 275alternating layers of Polymer 1 and Polymer 4. An additional set ofthick external protective skin layers made from Polymer 4 werecoextruded on either side of the 275 layer stack to form a cast web with277 total layers and a total thickness of 0.021 inches (0.53 mm). Inthis cast web, all layers were isotropic in refractive index, withPolymer 1 having an index of about 1.618 and Polymer 4 having an indexof about 1.567 at visible wavelengths. A piece of this cast web was thenheated by impingement with hot air at 100° C. for 45 seconds and thenoriented simultaneously in two orthogonal in-plane directions at a drawrate of 100%/sec to a final draw ratio of 3.6×3.6. The resulting opticalfilm had a thickness of about 0.0016 inches (0.041 mm) and a useablearea of about 10 by 10 inches (about 650 cm²). The refractive indices ofthe outer skin layers composed of Polymer 4 were measured with aMetricon prism coupler refractometer at 632.8 nm on the finished filmand found to be:

Polymer4: n_(x)=n_(y)=1.635; n_(z)=1.51

The refractive index of the other polymer was measured previously in acrushed pellet form with the same refractometer and found to be 1.618.By analyzing the optical properties of the finished (stretched) film,and knowing the final refractive indices of Polymer 4, it was determinedthat this other polymer had remained substantially isotropic, i.e., thatit had the following refractive indices in the finished film:

Polymer 1: n_(x)=n_(y)=n_(z)=1.618

Hence, for this film,

-   -   Δn_(x)=Δn_(y)≈0.017    -   Δn_(z)≈0.108        The relative thickness profile of the optical repeat units in        the finished film was measured with an atomic force microscope        (AFM). These relative measurements were then combined with a        global scaling factor and an f-ratio factor selected for best        agreement with the observed optical properties of the film, and        the resulting physical thickness profile of the 275 optical        layers in the film is shown in FIG. 6. Note that adjacent layers        have approximately the same physical thickness, and hence in        this case also approximately the same on-axis optical thickness        (f-ratio≈0.50). Also, multiple distinct nonzero layer thickness        gradients can be detected over various segments of the film        thickness. The individual optical layers range in physical        thickness from just under about 100 nm to about 125 nm. With the        refractive index properties of the two materials as noted above,        these thicknesses yield optical repeat units whose optical        thicknesses range from just under 325 nm to about 405 nm.        Doubling these values correspond to optical wavelengths of just        under 650 nm to about 810 nm.

Despite the fact that some of the optical repeat units had opticalthicknesses corresponding to half of a visible wavelength of light, thefilm was substantially clear when viewed at a normal angle (observationangle θ=0). When viewed at an observation angle of 60°, the film had amagenta transmitted appearance. This color was insensitive to rotationsof the film about an axis normal to the film. Further, the color couldbe made to be more saturated or could be made to substantially disappearusing an analyzer in front of the observer's eye, the analyzer beingrotated to transmit p-polarized light and s-polarized lightrespectively. Also, the film did not become cyan at any point betweenθ=0 and 60°. The percent transmission of the film in air was measuredand is shown in FIGS. 7 a and 7 b for normally incident light and lightincident at 60°, respectively. In FIG. 7 b, curve 60 is the transmissionof s-polarized light only, and curve 62 is for p-polarized light only.The FIG. 7 a-b graphs include no corrections or offsets for thebroadband surface reflections at the front and rear film-air interfaces.Note the absence of any substantial reflection bands at normalincidence. Note also the presence of a significant reflection band forp-polarized light in the visible region at 60° incidence. Theapproximately 50% broadband reflectivity of s-polarized light in FIG. 7b is due to the film-air surface reflections.

Additional Polymers and Film Embodiments

Additional polymers have been developed and/or identified that satisfythe conditions mentioned above: that they should be coextrudable; thatthey should have adequate interlayer adhesion; and that the isotropicpolymer should have an unusually high refractive index in order to matchthe in-plane refractive indices of the birefringent polymer afterstretching, and a low enough glass transition temperature so that itremains isotropic when oriented under conditions necessary to causebirefringence in the other polymer material. Further, the polymer usedas the isotropic layer desirably has a refractive index of at leastabout 1.61, more desirably about at least 1.65, so that polymers thatexhibit greater birefringence (e.g., pure PET) can be used to helpincrease the z-index differential between optical layers to achievehigher reflectivity.

Polymer 5—Co-PEN-5545HD

A copolyester was synthesized in a batch reactor with the following rawmaterial charge: 87.6 kg dimethyl naphthalene dicarboxylate, 57 kgdimethyl terephthalate, 12.3 kg hexane diol, 81.6 kg ethylene glycol,0.7 kg trimethylol propane, 34 g zinc acetate, 25 g cobalt acetate, and55 g antimony triacetate. Under pressure of 0.20 MPa, this mixture washeated to 254° C. while removing methanol. After 41.5 kg of methanol wasremoved, 56 g of triethyl phosphonoacetate was charged to the reactorand then the pressure was gradually reduced to 133 Pa while heating to285° C.

The condensation reaction by-product, ethylene glycol, was continuouslyremoved until a polymer with an intrinsic viscosity of 0.53 dL/g, asmeasured in 60/40 wt. % phenol/o-dichlorobenzene at 86° C., wasproduced. This material, a thermoplastic polymer, had a glass transitiontemperature T_(g) of 92° C. as measured by DSC using ASTM D3418 with ascan rate of 20° C./min, and at a relative humidity of about 50%. Thethermal history of the polymer was removed as a factor by performing twoDSC heat scans on the sample and recording the T_(g) of the second heatscan.

The polymer is suitable for use in the isotropic optical layers of amultilayer film, and has a refractive index of 1.612.

Polymer 6—Co-PEN Containing Nano-titania

The isotropic refractive index of coPEN-HNLT described as Polymer 1 canbe increased to 1.65 by incorporation of about 30 wt % titania particleswith average particle size of less than about 30 nm. Titania itself hasa refractive index of about 2.4 in the visible. The nano-titaniaparticles should be adequately dispersed to avoid excessive haze orscattering of light in the polymer matrix.

The resulting thermoplastic polymer-based material has the same glasstransition temperature as Polymer 1, i.e., about 76° C., and is suitablefor use in the isotropic optical layers of a multilayer film.

Polymer 7—Co-PEN Containing Nano-zirconia

The isotropic refractive index of coPEN-HNLT described as Polymer 1 canbe increased to 1.65 by incorporation of about 40 wt % zirconiaparticles with average particle size of less than about 30 nm. Zirconiaitself has a refractive index of about 2.2 in the visible. Thenano-zirconia particles should be adequately dispersed to avoidexcessive haze or scattering of light in the polymer matrix.

The resulting thermoplastic polymer-based material has the same glasstransition temperature as Polymer 1, i.e., about 76° C., and is suitablefor use in the isotropic optical layers of a multilayer film.

Polymer 8—High Index Acrylate Containing Nano-titania

Copolymers of naphthyl thio-acrylate and naphtyl thioethyl acrylateand/or naphthyl oxyethyl acrylate can be synthesized with 30 wt %titania particles to produce a polymer-based material having anisotropic refractive index of approximately 1.65. The titania particles,which should have an average size of less than about 30 nm, should beadequately dispersed to avoid excessive haze or scattering of light inthe acrylate polymer matrix.

The glass transition temperature of this polymer-based material can betailored by adjusting the relative proportions of the acrylate monomers,since T_(g)≈100° C. for naphthyl thio-acrylate, T_(g)≈40° C. for naphtylthioethyl acrylate, and T_(g)≈9° C. for naphthyl oxyethyl acrylate. Inparticular, the glass transition temperature of the material can betailored to be below 79° C., the glass transition temperature of PET.This Polymer 8 is suitable for use in the isotropic optical layers of amultilayer film.

Polymer 9—High Index Acrylate Containing Nano-zirconia

Copolymers of naphthyl thio-acrylate and naphtyl thioethyl acrylateand/or naphthyl oxyethyl acrylate can be synthesized with 40 wt %zirconia particles to produce a polymer-based material having anisotropic refractive index of approximately 1.65. The zirconiaparticles, which should have an average size of less than about 30 nm,should be adequately dispersed to avoid excessive haze or scattering oflight in the acrylate polymer matrix.

The glass transition temperature of this thermoplastic polymer-basedmaterial can be tailored by adjusting the relative proportions of theacrylate monomers, as described above in connection with Polymer 8, andcan be tailored to be below 79° C. This Polymer 9 is suitable for use inthe isotropic optical layers of a multilayer film.

Polymer 10—High Index Isotropic Co-PEN

A copolyester can be synthesized in a batch reactor with the followingraw material charge: 127.3 kg 2,6-dimethyl naphthalene dicarboxylate,8.4 kg 2,3-dimethyl naphthalene dicarboxylate, 48.4 kg hexane diol, 50.5kg ethylene glycol, 8.6 kg 1,3 butyl ethyl propanediol, 1.3 kgtrimethylol propane, 34 g zinc acetate, 25 g cobalt acetate, and 75 gantimony triacetate. Under pressure of 0.20 MPa, this mixture can thenbe heated to 254° C. while removing methanol. After 32.5 kg of methanolis removed, 56 g of triethyl phosphonoacetate can be charged to thereactor and then the pressure gradually reduced to 133 Pa while heatingto 285° C.

The condensation reaction by-product, ethylene glycol, can becontinuously removed until a polymer with an intrinsic viscosity of atleast 0.6 dL/g, as measured in 60/40 wt. % phenol/o-dichlorobenzene at86° C., is produced. This material, a thermoplastic polymer, has a glasstransition temperature T_(g) of approximately 76° C. as measured by DSCusing ASTM D3418 with a scan rate of 20° C./min, and at a relativehumidity of about 50%.

The thermoplastic polymer is suitable for use in the isotropic opticallayers of a multilayer film, and has a refractive index of 1.63.

Polymer 11—Co-PHT

A copolyester was synthesized in a batch reactor with the following rawmaterial charge: 100 kg dimethyl terephthalate, 93 kg 1,6-hexane diol,3.1 kg triethylene glycol, 0.9 kg trimethylol propane, 50 g tetra butlytitanate, 30 g cobalt acetate, and 35 g antimony triacetate. Underpressure of 0.20 MPa, this mixture was heated to 254° C. while removingmethanol. After 33 kg of methanol was removed, 35 g of triethylphosphonoacetate was charged to the reactor and then the pressure wasgradually reduced to 133 Pa while heating to 270° C.

The condensation reaction by-product, 1,6 hexane diol, was continuouslyremoved until a polymer with an intrinsic viscosity of 0.86 dL/g, asmeasured in 60/40 wt. % phenol/o-dichlorobenzene at 86° C., wasproduced. This material, a thermoplastic polymer, had a glass transitiontemperature T_(g) of 15° C. and a melting temperature T_(m) of 142° C.as measured by DSC using ASTM D3418 with a scan rate of 20° C./min, andat a relative humidity of about 50%. The thermal history of the polymerwas removed as a factor by performing two DSC heat scans on the sampleand recording the T_(g) of the second heat scan.

The polymer is suitable for use in the birefringent optical layers of amultilayer film, and has a pre-stretch refractive index of about 1.55.Under suitable biaxial stretching conditions, the in-plane refractiveindices can increase to about 1.59 to 1.61 and the out-of-planerefractive index can diminish to about 1.51.

Polymer 12—80/20 Polyester Blend

This is a blend of 80 wt % PET and 20 wt % PETG. It has a glasstransition temperature T_(g) of about 82° C.

The polymer is suitable for use in the birefringent optical layers of amultilayer film, and has a pre-stretch refractive index of about 1.568.Under suitable biaxial stretching conditions, the in-plane refractiveindices can increase to about 1.638 and the out-of-plane refractiveindex can diminish to about 1.506.

Polymer 13—CoPVN Isotropic Copolymer

Copolymers of vinyl naphthalene and phenoxy ethyl acrylate or other lowT_(g) acrylates such as ethyl acrylate, butyl acrylate, and iso-octylacrylate can be synthesized to provide a refractive index of 1.65 and aglass transition temperature of less than 79° C. Optionally, butadieneor other low T_(g) rubber comonomers can be copolymerized with vinylnaphthalate to provide a refractive index of 1.65 and a glass transitiontemperature of less than 79° C.

Polymer 14—Atactic PVN

Atactic polyvinyl naphthalene has an isotropic refractive index of 1.68and thus can be useful for increasing the index difference along thez-axis for increased reflectivity. The T_(g) of this material is 151°C., and hence it would be suitable for coextrusion and orientation witha higher T_(g) CoPEN as the birefringent material designed to havein-plane refractive indices of 1.68-1.70 after orientation.

Polymer 15—High T_(g) CoPEN Birefringent Polymer

Copolymers of PEN (polyethylene naphthalate) can be synthesizedutilizing 2,6 dimethyl naphthalate and 2,3 dimethyl naphthalate or 4,4biphenyl dicarboxylate as comonomers to dilute the in-plane refractiveindices of PEN down to 1.68-1.7 so as to match those of atactic PVN asthe istotropic material.

Further Film Embodiments

A multilayer optical polarizing film can be made using Polymer 5 as theisotropic light transmissible material, and Polymer 11 (the co-PHT) asthe birefringent light transmissible material. These materials can becoextruded through a multi-layer melt manifold to create a stack of 275(or other suitable number of) alternating layers of Polymer 5 andPolymer 11. An additional set of thick external protective skin layersmade from Polymer 11 can be coextruded on either side of the 275 layerstack to form a cast web with 277 total layers and a total thickness of,say, about 0.019 inches (0.48 mm) or other suitable value. In this castweb, all layers are isotropic in refractive index, with Polymer 5 havingan index of about 1.612 and Polymer 11 having an index of about 1.55 atvisible wavelengths. This cast web can then be heated to a suitabletemperature, such as 115° C., by impingement with hot air or otherconventional heating means and oriented simultaneously in two orthogonalin-plane directions at a suitable draw rate, such as 1000%/sec, to afinal draw ratio such as 3.0×3.0. The resulting optical film can have athickness of about 0.002 inches (0.05 mm). The skin and optical layerscomposed of Polymer 11 can achieve the following refractive indices inthe finished film:

Polymer 11: n_(x)=n_(y)=1.61; n_(z)=1.51

The refractive index of the other polymer can remain isotropic byappropriate selection of the stretch conditions, with an isotropicrefractive index of 1.612:

Polymer 5: n_(x)=n_(y)=n_(z)=1.612

Hence, for such a film,

-   -   Δn_(x)=Δn_(y)≈0.002    -   Δn_(z)≈0.102        The thickness profile of the optical layers in the finished film        can be any suitable function that achieves the desired        transmitted color at an oblique angle, whether uniform, step,        linear, or other known function.

Note that the in-plane refractive index difference is well below 0.01,yielding a film that is substantially clear when viewed at 0°observation angle even if each and every optical repeat unit in the filmhas an optical thickness of half of a visible wavelength.

In another embodiment, a multilayer optical polarizing film can be madeusing Polymer 1 as the isotropic light transmissible material, andPolymer 12 (the blend of 80 wt % PET and 20 wt % PETG) as thebirefringent material. These materials can be coextruded through amulti-layer melt manifold to create a stack of 223 (or other suitablenumber of) alternating layers of Polymer 1 and Polymer 12. The stackneed not have any layer thickness gradient, but preferably does have agradient corresponding to a full-width at half-maximum (FWHM) bandwidthof the reflection band in the finished film of about 100 nm. This stackcan be provided to an asymmetric multiplier where the extrudate is splitinto unequal widths of a suitable ratio, such as about 1:1.44, andstacked after equalizing the widths to provide two optical packets and atotal of 445 optical layers. An additional set of thick externalprotective skin layers made from Polymer 12 can be coextruded on eitherside of the 445 layers to form a cast web with 447 total layers and atotal thickness of 0.020 inches (0.51 mm). In this cast web, all layersare isotropic in refractive index, with Polymer 1 having an index ofabout 1.618 and Polymer 12 having an index of about 1.568 at visiblewavelengths. This cast web can then be heated by impingement with hotair at 102° C. and then oriented in two orthogonal in-plane directionsat a suitable draw rate to a final draw ratio of about 3.5×3.5. The skinand optical layers composed of Polymer 12 can achieve the followingrefractive indices in the finished film:

Polymer 12: n_(x)=n_(y)=1.638; n_(z)=1.506

The refractive index of the other polymer can remain isotropic byappropriate selection of the stretch conditions, with an isotropicrefractive index of 1.618:

Polymer 1: n_(x)=n_(y)=n_(z)=1.618

Hence, for such a film,

-   -   Δn_(x)=Δn_(y)≈0.02    -   Δn_(z)≈0.112        The resulting optical film can provide, for obliquely-incident        light, two distinct p-polarization reflection bands        corresponding to the two 223 layer packets in the film. With        suitable control of the layer thickness profile of each packet        and of the width ratio for the asymmetric multiplier, the        reflection bands can be separated sufficiently to define a gap        therebetween characterized by low reflectivity and high        transmission. In this way a film that shifts in transmitted        appearance from clear at normal incidence to green at about 60°        can be provided. Computed transmission spectra at 60° incidence        and for p-polarized light only are shown in FIG. 8, where the        curves 70, 72 are the computed transmission of the two        individual packets that make up the film. As demonstrated, each        packet produces a strong reflection band at the oblique angle.        The mathematical product of these two curves yields the computed        transmission of the film when illuminated with p-polarized light        at 60°. Of course, a clear-to-green multilayer film as just        described can also be made with other polymer combinations        taught herein, so long as the optical layer distribution is        tailored to provide the separated reflection bands at 60°.

In yet another embodiment, a multilayer optical interference film can bemade in the same way as the Example Film, except the layer thickness canbe controlled to provide a substantially linear layer thickness gradientacross the thickness of the film between limits that provide a singlereflection band extending from about 500 nm to about 600 nm forp-polarized light at 60°. Also, a yellow dye (absorbing from about 400to 500 nm) is incorporated into the film (or in a separate film orcoating laminated or otherwise applied to the multilayer film) in anamount sufficient to provide a yellow baseline color to the film atnormal viewing. FIG. 9 exemplifies the absorption of the yellow dye withcurve 76, which curve is relatively insensitive to changes in viewing orincidence angle θ. On the other hand, curve 78 exemplifies thetransmission of the optical layer stack for p-polarized light at θ=60°.Curve 78, of course, changes greatly with viewing angle, becoming a flatline at the upper end of the percent transmission scale for normallyincident light. Such a combination of absorbing dye and color shiftingp-polarizing film produces a film that changes from yellow at θ=0° tored at θ=60°.

Many variations of the foregoing yellow-to-red film are contemplated, bychanging the absorbing agent and/or the optical stack design (and thusthe reflection band position, width, number, and/or strength). In onesuch variation, the yellow dye is replaced with a blue dye that absorbsfrom about 600 to 700 nm. Also, the 500-600 nm reflection band isreplaced with band that extends from about 400 to 500 nm for p-polarizedlight at 60° by an appropriate change in optical layer thickness. Theresult is a film that changes from blue at θ=0° to green at θ=60°.

Other material pairs of interest for the p-polarizing multilayer opticalfilm are combinations in which the birefringent light transmissiblematerial is Polymer 2 (PET), and the isotropic light transmissiblematerial is selected from the group of Polymer 6 (co-PEN withnano-titania), Polymer 7 (co-PEN with nano-zirconia), Polymer 8(acrylate with nano-titania), and Polymer 9 (acrylate withnano-zirconia). These material combinations can be coextruded andoriented under suitable conditions analogous to those of the Example toprovide a finished multilayer p-polarizing film whose layers have thefollowing refractive indices:

-   -   Isotropic material: n_(x)≈n_(y)≈n_(z)≈1.65    -   Birefringent material (Polymer 2): n_(x)≈n_(y)≈1.65; n_(z)≈1.49        And thus,    -   Δn_(x)≈Δn_(y)≈0    -   Δn_(z)≈0.16        The relatively large z-index difference—greater than        0.15—provides substantially higher reflectivity for off-axis        p-polarized light. At the same time, a good in-plane index match        ensures substantially no reflection bands at normal incidence.

Another material pair of interest for the p-polarizing multilayeroptical film is a combination in which the birefringent lighttransmissible material is Polymer 12 (80% PET/20% PETG) and theisotropic light transmissible material is Polymer 10 (High Indexco-PEN). These materials can be coextruded and oriented under suitableconditions analogous to those of the Example to provide a finishedmultilayer p-polarizing film whose layers have the following refractiveindices:

-   -   Polymer 10: n_(x)≈n_(y)≈n_(z)≈1.63    -   Polymer 12: n_(x)n_(y)≈1.64; n_(z)≈1.50        And thus,    -   Δn_(x)=Δn_(y)≈0.01    -   Δn_(z)≈0.13

In other embodiments, sPS (syndiotactic polystyrene) or sPN(syndiotactic polynaphthalene) can be utilized as the birefringentmaterials. Since these polymers are characterized in that refractiveindices along the stretch directions (x- and y-axes) decrease and therefractive index along the z-axis increases upon orientation, theisotropic polymer should be chosen such that its refractive indices arelower than that of either sPS or sPN. For example, the refractiveindices of sPS are 1.585 before orientation and after stretching thein-plane refractive indices derease to 1.56 and the z-axis refractiveindex increases to 1.65. Since the T_(g) of sPS is approximately 105°C., a copolymer such as PETG can be used as the isotropic polymer togive the following sets of refractive indices after orientation:

-   -   PETG: n_(x)≈n_(y)≈n_(z)≈1.56    -   sPS: n_(x)≈n_(y)≈1.56; n_(z)≈1.65    -   Δn_(x)≈Δn_(y)≈0.0    -   Δn_(z)≈0.09        Glossary of Certain Terms

-   F-ratio: the relative contribution of a given individual layer to    the total optical thickness of a given ORU. The f-ratio for the k-th    individual layer is:    ${f_{k} = \frac{n_{k} \cdot d_{k}}{\sum\limits_{m = 1}^{N}\quad{n_{m} \cdot d_{m}}}},$    where 1≦k≦N, where N is the number of constituent layers in the ORU,    where n_(k) (n_(m)) is the relevant refractive index of k-th (m-th)    layer, and d_(k) (d_(m)) is the physical thickness of layer k (m).

-   Optical Repeat Unit (“ORU”): a stack of at least two individual    layers which repeats across the thickness of a multilayer optical    film, though corresponding repeating layers need not have the same    thickness.

-   Optical thickness: the physical thickness of a given body times its    refractive index. In general, this is a function of wavelength and    polarization.

-   Reflection band: a spectral region of relatively high reflectance    bounded on either side by regions of relatively low reflectance.

All patents and patent applications referenced herein are incorporatedby reference in their entirety. Various modifications and alterations ofthis invention will be apparent to those skilled in the art withoutdeparting from the scope and spirit of this invention, and it should beunderstood that this invention is not limited to the illustrativeembodiments set forth herein.

1. An optical film, comprising a plurality of layers that form aplurality of optical repeat units, at least some of the optical repeatunits having optical thicknesses equal to half of a wavelength ofvisible light, and wherein the optical film has a clear appearance atnormal incidence.
 2. The film of claim 1, wherein the film has noreflection band of reflectivity greater than 20% for normally incidentlight.
 3. The film of claim 1, wherein the film comprises alternatinglayers of a first and second thermoplastic polymer, and wherein thefirst polymer is substantially isotropic and the second polymer isbirefringent.
 4. The film of claim 3, wherein the second polymer has arefractive index along a thickness axis of the film that differs from arefractive index of the first polymer along the thickness axis of thefilm by at least 0.1.
 5. The film of claim 3, wherein the first polymerhas a refractive index of at least 1.65.
 6. The film of claim 3, whereinthe film has a first and second portion, and the alternating layers inthe first portion have a different overall thickness than thealternating layers in the second portion.
 7. The film of claim 1,wherein the film has a clear transmitted appearance at normal incidence.8. An optical film whose transmitted appearance changes fromsubstantially clear to a first color over an angular range ofobservation angles, and where the optical film does not appear cyan oversuch angular range.